1. Field of the Invention
This invention relates to thin film emitter structures in field emission devices and more specifically thin film emitter structures in vacuum microelectronic three terminal devices.
22. Description of Related Art
Vacuum microelectronic three terminal devices, typically described as vacuum transistors or vacuum triodes, use a technology associated with electron transport in vacuum and are fabricated with modern microfabrication technology developed for solid state devices. Microelectronic triodes are built using thin film techniques and operate similarly to vacuum tube triodes while utilizing integrated circuit and micromachining techniques for fabrication. As a result, microelectronic triodes have a number of significant advantages over their predecessors. These include having a wider operable temperature range, higher efficiency performance, smaller and lighter weight packaging, higher resistance to radiation damage and a lower manufacturing cost.
Three terminal devices, such as the microelectronic triode, generally include three elements: a single cathode, a control electrode and an anode, although there may be variations having pluralities of any of these parts. The control electrodes act as a gate which controls the current flow between the anode and the electron-emitting cathode. Such devices are used in microwave and millimeter frequency applications requiring large power, such as in the utilization of active antenna arrays in electronic countermeasures, radar and communication systems.
Field emission devices have the problem of being unreliable. The electron-emitting cathodes, or emitters, often experience a problem with d.c. burn-out at their emitting edges. Burn-out is a phenomenon which occurs when excessive current is run through the emitter. Studies on the failure of field emitters are documented in papers by Ivor Brodie, "Bombardment of Field-emission Cathodes by Positive Ions Formed in the Interelectrode Region", Int. J. Electronics, Vol. 38, No. 4, 1975 and Jim Browning, Nicol E. McGruer, W. J. Bintz, and M. Gilmore, "Experimental Observations of Gated Field Emitter Failures", IEEE Electron Device Letters, Vol. 13, No. 13, Mar. 1992. Burn-out initiates at a small point on the emitter and eventually spreads and burns out the entire emitter, making the entire device inoperable.
Excessive current leading to burn-out at the emitter edge normally occur in two instances. The first instance is when there are several "whiskers" or sharp points on the surface of the emitter structure. Sharp points are points on the emitter where there is a concentrated electric field which attracts current. When there are too many sharp points on the emitter burn-out occurs.
The second instance when burn-out occurs is when energy in stray capacitances discharge uncontrollably at the emitter. The discharge rate of capacitance generally depends on the value of the resistance in series with it. When a large charge build-up is discharged over a small resistance in a microelectronic triode, there is a danger that the high current will cause burn-out.
In the past, large resistors were placed in series between the lead-in conductor and the emitter to solve the emitter edge burnout problem. As a result, d.c. current to the emitter would be limited for any given applied voltage by the resistance. This arrangement achieved the purpose of limiting the emission current. It would also, however, produce the undesirable result of degrading the performance of the emitter by lowering its transconduction constant.